The invention generally relates to oxide charge measurements on microelectronic device chips and, more particularly to making such measurements on VLSI product chips characterized by high device density, relatively thick oxides and relatively high substrate doping levels in the field regions.
The well-known capacitance versus bias voltage technique for measuring oxide charge is not well suited for use with present day high device density chips where permissible probe size is very limited due to access space constraints in the oxide-on-silicon regions of the chips. The limited probe area reduces the signal-to-noise ratio of the resulting data signal for deriving the desired oxide charge measurement. Moreover, this situation is exacerbated in the case of increasing oxide thickness and increasing silicon doping concentration, typical of the accessible VLSI measurement sites, whereby the maximum and minimum capacitance of a C-V trace tend towards equalling each other. Thus, the problem becomes one of trying to amplify the difference between two capacitances that are noisy and approximately of the same magnitude. The result is then a signal which is too weak and too noisy to be useful.
It is also known to employ a photovoltaic effect for making contactless measurements at the surface of semiconductor devices utilizing a variable bias voltage on a capacitive pick-up. A transparent electrode (for allowing light to pass through) and an insulator or air gap between the electrode and the underlying semiconductor is usually employed. Such pick-up probes require that relatively extensive illumination and surrounding capacitive pick-up areas be available on the devices undergoing measurements. U.S. Pat. No. 5,087,876 issued to Leszek Reiss, et al. on Feb. 11, 1992, U.S. Pat. No. 5,025,145 issued to Jacek J. Lagowski on Jun. 18, 1991, U.S. Pat. No. 4,827,212 issued to Emil Kamieniecki on May 2, 1989, U.S. Pat. No. 4,464,627 issued to Chusuke Munakata, et al. on Aug. 7, 1984, IBM.RTM. Technical Disclosure Bulletin article entitled "Contactless Photovoltage vs Bias Method for Determining Flat-Band Voltage", by M. S. Fung, et al. Vol. 32, No. 9A, February, 1990, p. 14 and IBM Technical Disclosure Bulletin article entitled "Rapid Contactless Method for Measuring Fixed Oxide Charge Associated With Silicon Processing" by R. L. Verkuil, Vol. 24, No. 6, November, 1981, all are examples of the aforedescribed photovoltaic effect technology.
U.S. Pat. No. 4,956,603 issued to Vincenzo Russo on Sep. 11, 1990 discloses a method for measuring the lifetime of a PN semiconductor junction which includes subjecting one side of the junction to light of a known intensity and measuring the voltage generated by the photovoltaic effect at the ends of the junction. The method is not suitable for oxide charge measurement in LSI wafers.